Listeria monocytogene (LM) is a stimulator of the RES recently used as a non-specific promoter of tumor cell death in vivo and in vitro. The present studies are designed towards purification, identification and analysis of cellular collaboration synergism and/or antagonism during tumor cell destruction. Macrophages and specifically sensitized thymus derived (T) cells interact to promote or prevent cell death in vitro. Tumor cell death is extracellular and does not involve contact or adhesion between tumor and effector cells, is non-phagocytic and apparently non-enzyme mediated. The role of B-cells, nul cells, other accessory cells and soluble mediators are being investigated. These and similar work in vivo are being conducted for a better understanding of mechanism involved in potentiating host cellular response to neoplasia.